The Prior Art
Such compounds are known to possess biocidal and biostatic activity towards a variety of organisms. The isothiazolinones of this type, however, are often not obtained in free form, but as complexes of the formula (2): ##STR2## wherein M is a metallic or an amino cation, X is an anion forming a compound with the cation M, and the value of n is such that the anion X.sub.n satisfies the valence of M. The complexes of formula (2), described in U.S. Pat. No. 4,067,878, are said to be more stable than the 3-isothiazolones of formula (1).
The term alkyl group for the substituents R and Y is intended to include both unsubstituted or substituted alkyl, alkenyl, cycloalkyl, aralkyl or aryl groups. In a preferred embodiment the alkyl group is selected from methyl and n-octyl.
Examples of the cation M are metal cations such as calcium, copper, magnesium, manganese, nickel, sodium, potassium and zinc and complexes of the metal ions such as complexes with ammonia and amines.
For bactericidal and fungicidal purposes, particularly useful compositions, described in U.S. Pat. No. 4,105,431, comprise a mixture N-alkyl-isothiazolin-3-one and N-alkyl-5-chloro-isothiazolin-3-one, in a weight ratio of about 1:3 respectively. A particularly useful mixture of this kind is that in which Y is a methyl group, and such a mixture is referred to hereinafter as MIT (methylisothiazolinones).
Formulations of MIT in water or in solvent media containing hydroxylic groups are unstable, decompose rapidly and cannot be stored for long periods of time. The art has searched for ways to overcome this stability problem for a long time. A solution suggested in the art comprises stabilizing 3-isothiazolinones either in liquid formulations or on solid supports, by the addition of metal nitrates. Some such methods are described, e.g., in U.S. Pat. Nos. 3,870,795, 4,067,878, EP 0 106 563 and EP 0 166 611. Such methods have the considerable drawback of requiring the addition of metal nitrates in amounts which are usually nearly equimolar--but often even in excess--with respect to the 3-isothiazolinones. MIT, being a very effective biocide, is often required in application in very low concentrations of the active ingredient (a.i.). For such purposes, it is more convenient to provide MIT formulations of low a.i. concentrations. However, commercially available formulations stabilized only by large concentrations of nitrate salts become less stable on dilution. Furthermore, the addition of nitrates may lead to the presence of nitrosamines, which are highly undesirable impurities which are suspected of being carcinogens. Therefore, some of the methods of the art have the added disadvantage of requiring means for removing such nitrosamines or their precursors, as disclosed, e.g., in EP 0 095 907, or for inhibiting their formation. Such operations are complicated, time-consuming and do not afford the certainty. that a sufficient amount of nitrosamines or of their precursors has been removed.
Another disadvantage in the use of metal nitrate stabilization is that in the application of such formulae for the protection of latex-based products (e.g., in paints), these salts may cause undesirable coagulation. This is particularly true in the case when di- and polyvalent metallic ions are present in the MIT formulation.
An effective method for stabilizing MIT is described in U.S. Pat. No. 4,920,137 of the same inventors, which is characterized in that a stabilizing effective amount of a stabilizing compound of the formula (3): EQU [R.sub.x A--C.sub.6 H.sub.2 R.sup.1 R.sup.2 ].sub.y --Z (3)
wherein:
R, R.sup.1 and R.sup.2 each independently represents hydrogen, a straight-chained or branched or cyclic alkyl radical, aralkyl or aryl; PA1 A is oxygen or nitrogen; PA1 Z represents AR.sub.x, R.sup.1, R.sup.2, alkoxy methylene, methylene or alkylidene; provided that when A is oxygen, x is 0 or 1 and when A is nitrogen, x is 1 or 2; and y is 1 or 2; PA1 R, R.sup.1, R.sup.2, A, Z, x and y have the meanings described above.
is added to the composition containing a 3-isothiazolinone or two or more 3-isothiazolinones.
A preferred group of stabilizers, which possess enhanced stabilization properties, consists essentially of hydroquinone, quinone and quinhydrone, and their derivatives and homologues. Other representative stabilizing compounds are, e.g., tert-butylcatechol, p-methoxyphenol, and p-phenylenediamine and its derivatives.
According to a preferred embodiment of the said invention, the composition to be stabilized comprises a mixture of N-alkyl-isothiazolin-3-one and N-alkyl-5-chloro-isothiazolin-3-one. Particularly useful mixtures of this kind are those in which the N-alkyl radical is a N-methyl radical.
The said patent is also directed to stable compositions containing one or more 3-isothiazolinones and a stabilizing effective amount of a compound of the formula (3): EQU [R.sub.x A--C.sub.6 H.sub.2 R.sup.1 R.sup.2 ].sub.y --Z (3)
wherein:
While the presence of metal nitrate stabilizers in the compositions is not necessary, such nitrate stabilizers could be also added, together with the said stabilizing compound(s). Adding to the composition to be stabilized also one or more metal salt(s), selected from metal nitrate stabilizers and salts of metals of groups IA and IIA of the periodic table of the elements, provides a synergistic stabilizing effect, and may be convenient in some cases. In any case, the synergistically effective amount of metal nitrate stabilizers employed in any given composition of U.S. Pat. No. 4,920,137 can be much lower than amounts used in the known art, and hence the amount of nitrosamines or nitrosamine precursors would be drastically reduced.
By "synergistically effective amounts" is meant any amount which, while by itself incapable of effectively stabilizing 3-isothiazolinones, improves the stabilization of 3-isothiazolinone formulations which is provided by the sole addition of stabilizing effective amounts of stabilizing compounds of formula (3).
Preferred metal salts are selected from the group consisting of magnesium nitrate, K.sub.2 HPO.sub.4, KH phthalate, magnesium acetate, NaNO.sub.3, KNO.sub.3 and potassium permanganate.
Preferred stabilizing compounds comprise hydroquinone, quinone and quinhydrone, and their derivatives and homologues.